An image or characters may be superimposed on an image signal as special effect processing. In this case, a key signal is used that specifies the shape (range) of an image to be cut out for superimposition and the amount (intensity) of the image.
If an image contains, for example, a human being against a blue background (such an image is referred to as a 0chromakey), the key signal distinguishes that human being from the blue background. The key signal specifies a cut-out image area (for example, the area of the human being) as "1" and the blue background as "0". Edges are added to the periphery of the cut-out image area in the key signal.
A conventional edge key generating apparatus is configured as shown in FIG. 8.
In this figure, 30 is an input terminal through which an image signal for superimposition is input; 31 is a Y/C separation circuit that separates the image signal (in which, for example, a luminance signal (hereafter referred to as "Y" ) and a color signal (hereafter referred to as "C") are time-axis-multiplexed) into Y and C; 32 is a key signal generating circuit that generates a key signal based on the weights .of the respective signals provided by the Y/C separating circuit 31; 33 is a filter that removes ringing from the key signal; 34 is an edge generating circuit that generates (adds) edges of a desired width to the key signal from which the filter 33 has cut out harmonic components; and 35 is an output terminal through which the key signal is output to which the edges have been added.
For character superimposition, the key signal generating circuit 32 generates a key signal based on the level of Y, and if the characters have colors, a key signal is generated taking the color signal into consideration.
The edge generating circuit 34 is configured as shown in FIG. 9.
FIG. 9 is a simplified drawing of a configuration used to generate edges in the vertical direction. A key signal output from a filter 33 is input through a terminal 36. In a 1H delay circuit 37 that delays a signal by one horizontal scanning period (hereafter referred to as "1H"), the key signal is delayed by 1H. A 1H delay circuit 38 further delays the signal by 1H. Signals for three lines from the terminal 36 and 1H delay circuits 37 and 38 are input to a maximum-value selector 39 to select a signal having a maximum amplitude level, which is then output from a terminal 40.
If, for example, a key signal (the shaded part) is in the n-th line ((n) is an integer) of an image signal as shown in FIG. 10a, then in the output from the edge generating circuit 34 shown in FIG. 9, an edge is added to one line located above and below the n-th line, respectively as shown in FIG. 10b.
Specifically, in the configuration shown in FIG. 9, the edge generating circuit 34 provides a 1H delay, so when a signal for the n-th line is input to the terminal 36, the (n-1)-th line is output from the 1H delay circuit 37 and the (n-2)-th line is output from the 1H delay circuit 38. Thus, as the output of the signal of the (n-1)-th line from the maximum-value selector 39, the signal from the terminal 36, that is, the key signal is selected.
Likewise, when the (n+1)-th line is input to the terminal 36, selection is made among (n+1)-th, (n)-th, and (n-1)-th lines, and when the (n+2)-th line is input to the terminal 36, selection is made among the (n+2)-th, (n+1)-th, and (n)-th lines.
An example of a configuration of an actual edge generating circuit 34 is shown in FIG. 11.
The configuration shown in FIG. 11 can add edges to four lines above and below the key signal. A key signal is input through a terminal 41 and is sequentially delayed by 1H delay circuits 43 to 50 via a one-field FIFO memory having one field of storage capacity in order to provide signals for nine lines.
In this circuit, the output from the 1H delay circuit 46 is used as a reference line, so the signal is delayed by 4H relative to the output from the one-field FIFO memory 42. The key signal with edge addition must be timed to an image signal for superimposition. Thus, the one-field FIFO memory is adjusted to delay the signal by one-field period including the 4H delay.
The outputs from the one-field FIFO memory 42, 1H delay circuits 43 to 45, and 1H delay circuits 47 to 50 are multiplied by .alpha.1 to .alpha.8 by multipliers 51 to 58 respectively. The outputs from the multipliers 51 to 58 and 1H delay circuit 46 are input to a maximum-value selection circuit 59 to select one of the signals for nine lines which has a maximum amplitude, which is output from a terminal 61 as the key signal to which edges have been added.
A control signal is input to the maximum-value selector 59 from a terminal 60. The control signal controls the method for adding edges, that is, specifies the number of edges to be added and the addition of both upper and lower edges, only upper edges, only lower edges, or no edges. Selection among the signals for nine lines causes four edges to be added above and below the key signal, and selection among the 1H delay circuits 46 to 50 causes four edges to be added below (above?) the key signal.
In addition, coefficients .alpha.1 to .alpha.8 applied to the multipliers 51 to 58 control the method for adding edges. When, for example, .alpha.1 =0.2, .alpha.2 =0.4, .alpha.3 =0.6, .alpha.4 =0.8, .alpha.5 =0.8, .alpha.6 =0.6, .alpha.7 =0.4, and .alpha.8 =0.2, soft-edge processing is executed in which the edges added before and after the key signal (in the upper and lower parts of an image) vary on a step-by-step basis as shown in FIG. 12a. In addition, if .alpha.1 to .alpha.8 are all 1, hard edge processing is executed in which the edges added before and after the key signal are all at the same level as shown in FIG. 12b.
Since such a conventional edge key generating apparatus is designed to generate edges for an image signal for interlaced scanning as in NTSC image signals, it is not affected by gaps between edges caused by the absence of the key signal in the subsequent field as shown in FIG. 10.
Attempts, however, are now made to put to practical use television broadcasting using progressive scanned image signals. A progressive scanned image signal carries one frame of image in one conventional field, thereby, for example, doubling the number of required horizontal scanning lines.
Consequently, to reduce the required transmission band, broadcasting studios use a transmission method that separates a progressive image signal into two.
FIG. 13 shows a separation method used in transmitting a progressive image signal that is separated into two. In this figure, a frame (for example, one frame lasts one-thirtieth seconds) is divided into links A and B (a main link and a sub-link) for each horizontal scanning line. When one frame corresponds to 525 horizontal scanning lines, the positions of the links A and B are reversed during the subsequent frame. Thus, if only the signal of the link A or link B is considered, it is the same as an interlaced image signal so a conventional apparatus for interlaced image signals can be used in this case.
If, however, a conventional edge key generating apparatus is applied to each of the links A and B, although all the signals are contained in the same frame, gaps between the edges are clearly recognized, resulting in striped edges.
If, for example, the signal shown in FIG. 10 is assumed to be a progressive image signal, the lines (n) and (n+1) are signals of the link A, whereas the line between these lines is a signal of the link B within the same frame. As a result, if edges are added to the signals of the links A and B using an edge key generating apparatus, followed by multiplexing, then striped edges result as shown in FIG. 10b.